Hi,
You can rename the XLSX file to ZIP extension, then decompress the zip file.
Go to the /xl/sharedStrings.xml and check the line and column showed in the error message. Probably there is an strange character that Excel dont recognise.
You also can check encoding at the head of the file and try with other encodings:
<?xml version="1.0" encoding="UTF-8"?>
<?xml version="1.0" encoding="UTF-16"?>
<?xml version="1.0" encoding="iso-8859-1"?>
<?xml version="1.0" encoding="windows-1252"?>
etc.
When you have deleted the strange character (or changed the encoding), you can save the file, compress again to ZIP and rename it to XLSX.
Regards,
Juan Miguel